Download Exponential Growth

Teresa Audesirk • Gerald Audesirk • Bruce E. Byers
Biology: Life on Earth
Eighth Edition
Lecture for Chapter 26
Population Growth
and Regulation
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 26 Outline
• 26.1 How Does Population Size Change? p.
514
• 26.2 How Is Population Growth Regulated? p.
515
• 26.3 How Are Populations Distributed in Space
and Time? p. 524
Section 26.1 Outline
• 26.1 How Does Population Size
Change?
– Biotic Potential Can Produce Exponential
Growth
The Study of Ecology
• Ecology: the study of interrelationships
between living things and their nonliving
environment
• The environment consists of two
components
– Abiotic component: nonliving, such as soil
and weather
– Biotic component: all living forms of life
The Study of Ecology
• Ecology can be studied at several
organizational levels:
– Populations: all members of a single species
living in a given time and place and actually
or potentially interbreeding
– Communities: all the interacting populations
in a given time and place
– Ecosystem: all the organisms and their
nonliving environment in a defined area
– Biosphere: all life on Earth
How Does Population Size Change?
• Several processes can change the size of
populations
– Birth and immigration add individuals to a
population
– Death and emigration remove individuals
from the population
• Change in population size
= (births – deaths) + (immigrants – emigrants)
How Does Population Size Change?
• Ignoring migration, population size is
determined by two opposing forces
– Biotic potential: the maximum rate at which
a population could increase when birth rate is
maximal and death rate minimal
– Environmental resistance: limits set by the
living and nonliving environment that
decrease birth rates and/or increase death
rates (examples: food, space, and predation)
Exponential Growth
• Exponential growth occurs when a
population continuously grows at a fixed
percentage of its size at the beginning of
each time period
– This results in a J-shaped growth curve
• Doubling time describes the amount of
time it takes to double its population at its
current state of growth
FIGURE 26-1a Exponential
growth curves are J-shaped
All such curves share a similar J
shape; the major difference is
the time scale. (a) Growth of a
population of bacteria, starting
with a single individual and with
a doubling time of 20 minutes.
Biotic Potential
•
Biotic potential is influenced by several
factors
(1) The age at which the organism first
reproduces
– Populations that have their offspring earlier
in life tend to grow at a faster rate
Biotic Potential
(2) The frequency at which reproduction occurs
Biotic Potential
(3) The average number of offspring
produced each time
(4) The length of the organism's
reproductive life span
(5) The death rate of individuals
– Increased death rates can slow the rate of
population growth significantly
FIGURE 26-2 The
effect of death rates
on population growth.
The graphs assume
that a bacterial
population doubles
every 20 minutes.
Notice that the
population in which a
quarter of the
bacteria die every 20
minutes reaches
2500 only 2 hours
and 20 minutes later
than one in which no
deaths occur.
Section 26.2 Outline
• 26.2 How Is Population Growth
Regulated?
– Exponential Growth Only Occurs Under
Special Conditions
– Environmental Resistance Limits Population
Growth
Exponential Growth
• Exponential growth cannot continue
indefinitely
• All populations that exhibit exponential
growth must eventually stabilize or crash
• Exponential growth can be observed in
populations that undergo boom-and-bust
cycles
– Periods of rapid growth followed by a sudden
massive die-off
Exponential Growth
• Boom-and-bust cycles can be seen in
short lived, rapidly reproducing species
– Ideal conditions encourage rapid growth
– Deteriorating conditions encourage massive
die-off
• Example
– Each year cyanobacteria in a lake may
exhibit exponential growth when conditions
are ideal, but crash when they have depleted
their nutrient supply
FIGURE 26-3 A boom-and-bust population cycle
Population density of cyanobacteria (blue-green algae) in an annual boom-andbust cycle in a lake. Algae survive at a low level through the fall, winter, and
spring. Early in July, conditions become favorable for growth, and exponential
growth occurs through August. Nutrients soon become depleted, and the
population "goes bust."
Exponential Growth
• Example
– Lemming cycles are more complex and
involve overgrazing of food supply, large
migrations, and massive mortality caused by
predators and starvation
FIGURE 26-4 Lemming population cycles follow a boom-and-bust pattern
Lemming population density follows roughly a four-year cycle (data from Point
Barrow, Alaska)
Exponential Growth
• Temporary exponential growth can occur
when population-controlling factors are
relaxed, such as
– When food supply is increased
– When predators are reduced
• When exotic species are introduced into
a new ecosystem, population numbers
may explode due to lack of natural
predators
Exponential Growth
• When species are protected, e.g. the
whooping crane population has grown
exponentially since they were protected
from hunting and human disturbance in
1940
FIGURE 26-5
Exponential growth
of wild whooping
cranes
Hunting and habitat
destruction had
reduced the world's
whooping crane
population to about
20 before they were
protected in 1940.
By 2005, their wild
population had
grown to 340
individuals. Notice
the J-curve
characteristic of
exponential growth.
Environmental Resistance
• Many populations that exhibit exponential
growth eventually stabilize
• Environmental resistance limits population
growth
– As resources become depleted, reproduction
slows
Environmental Resistance
• This growth pattern, where populations
increase to the maximum number
sustainable by their environment, is called
logistic growth
• When this growth pattern is plotted, it
results in an S-shaped growth curve (or Scurve)
FIGURE 26-6a The S-curve of logistic population growth
(a) During logistic growth, the population will remain small for a time, then will
expand increasingly rapidly for a time. Then the growth rate slows and growth
eventually ceases at or near the carrying capacity (K). The result is a curve
shaped like a "lazy s."
Environmental Resistance
• Carrying capacity (K) is the maximum
population size that can be sustained by
an ecosystem for an extended time
without damage to the ecosystem
Environmental Resistance
• Logistic population growth can occur in nature
when a species moves into a new habitat, e.g.
barnacles colonizing bare rock along a rocky
ocean shoreline
• Initially, new settlers may find ideal conditions
that allow their population to grow almost
exponentially
• As population density increases, individuals
compete for space, energy, and nutrients
Environmental Resistance
• These forms of environmental resistance
can reduce the reproductive rate and
average life span and increase the death
rate of young
• As environmental resistance increases,
population growth slows and eventually
stops
FIGURE 26-8 A logistic curve in nature
Barnacles are crustaceans whose larvae are carried in ocean
currents to rocky seashores where they settle and then attach
permanently to rock and grow into the shelled adult form. On a
bare rock, the number of settling larvae produce a logistic growth
curve as competition for space limits their population density.
Environmental Resistance
• If a population far exceeds the carrying
capacity, excess demands decimate
crucial resources
• This can permanently and severely
reduce K, causing the population to
decline to a fraction of its former size or
disappear entirely
FIGURE 26-7 The effects of exceeding carrying capacity
Exceeding carrying capacity can damage an ecosystem, reducing its ability to
support the population. In 1911, 25 reindeer were introduced onto one of the
Pribilof Islands (St. Paul) in the Bering Sea off Alaska. Food was plentiful, and
the reindeer encountered no predators on the island. The herd grew
exponentially (note the initial J shape) until it reached 2000 reindeer in 1926. At
this point, the small island was seriously overgrazed, food was scarce, and the
population declined dramatically. By 1950, only eight reindeer remained.
Environmental Resistance
• In nature, conditions are never completely
stable, so both K and the population size
will vary somewhat from year to year
• However, environmental resistance ideally
maintains populations at or below the
carrying capacity of their environment
Environmental Resistance
• Environmental resistance can be
classified into two broad categories
– Density-independent factors
– Density-dependent factors
Density-Independent Factors
• Density-independent factors limit
populations regardless of their density
– Examples: climate, weather, floods, fires,
pesticide use, pollutant release, and
overhunting
• Some species have interesting means of
limiting their losses
– Examples: seasonally migrating to a better
climate or entering a period of dormancy
when conditions deteriorate
Density-Dependent Factors
• Density-dependent factors become
more effective as population density
increases
• Exert negative feedback effect on
population size
• Can cause birth rates to drop and/or death
rates to increase
FIGURE 26-9 Density-dependent environmental resistance
In response to crowding, laboratory populations of fruit flies show a decrease in
both reproductive rate and life span. In this graph, population density (horizontal
axis) increases from left to right. Notice that the number of offspring produced
per day decreases as population density increases. The life span remains
relatively constant until population density reaches a critical level, causing life
span to drop off dramatically.
Density-Dependent Factors
• At carrying capacity, each individual's
share of resources is just enough to allow
it to replace itself in the next generation
• At carrying capacity birth rate = death rate
• Carrying capacity is determined by the
continuous availability of resources
Density-Dependent Factors
• Include community interactions
– Predation
– Parasitism
– Competition
Predation
• Predation involves a predator killing a
prey organism in order to eat it
– Example: a pack of grey wolves hunting an
elk
• Predators exert density-dependent
controls on a population
– Increased prey availability can increase birth
rates and/or decrease death rates of
predators
• Prey population losses will increase
FIGURE 26-10 Predators help control prey populations
A pack of grey wolves has brought down an elk that may have
been weakened by age or parasites
Predation
• There is often a lag between prey
availability and changes in predator
numbers
– Overshoots in predator numbers may cause
predator-prey population cycles
– Predator and prey population numbers
alternate cycles of growth and decline
Predation
• Predation may maintain prey populations
near carrying capacity
– “Surplus" animals are weakened or more
exposed
• Predation can also maintain prey
populations well below carrying capacity
– Example: the cactus moth used to control
exotic prickly pear in Australia
Parasitism
• Parasitism involves a parasite living on
or in a host organism, feeding on it but
not generally killing it
– Examples: bacterium causing Lyme disease,
some fungi, intestinal worms, ticks, and some
protists
Parasitism
• While parasites seldom directly kill their
hosts, they may weaken them enough that
death due to other causes is more likely
• Parasites spread more readily in large
populations, and so will have a greater
effect acting as a ‘population control’ on
larger populations.
Competition for Resources
• Competition
– Describes the interaction among individuals
who attempt to utilize a resource that is
limited relative to the demand for it
• Competition intensifies as populations
grow and near carrying capacity
• For two organisms to compete, they must
share the same resource(s)
Competition for Resources
• Competition may be divided into two
groups based on the species identity of
the competitors
– Interspecific competition is between
individuals of different species
– Intraspecific competition is between
individuals of the same species
Competition for Resources
• Competition may also be divided into two
types based on the nature of the
interaction
– Scramble (exploitative) competition is a
free-for-all scramble as individuals try to beat
others to a limited pool of resources
– Example: Gypsy moth caterpillars
FIGURE 26-11
Scramble competition
Gypsy moths gather on
tree trunks to lay egg
masses that each
produce many
hundreds of
caterpillars (inset).
Competition for Resources
• Competition may also be divided into two
types based on the nature of the
interaction
– Contest (interference) competition involves
social or chemical interactions that limit a
competitor’s access to resources
Competition for Resources
• Intense local competition may drive
organisms to emigrate, though mortality
may be intense
– Example: swarming in locusts
Factors Interact
• The size of a population at any given time
is the result of complex interactions
between density-independent and densitydependent forms of environmental
resistance
Section 26.3 Outline
• 26.3 How Are Populations Distributed
in Space and Time?
– Populations Exhibit Different Spatial
Distributions
– Survivorship in Populations Follows Three
Basic Patterns
Spatial Distributions
• The spatial pattern in which individuals
are dispersed within a given area is that
population’s distribution, which may vary
with time
• There are three major types of spatial
distributions
– Clumped
– Uniform
– Random
Spatial Distributions
• Clumped distribution – includes family and
social groups
• Examples: elephant herds, wolf packs, prides of
lions, flocks of birds, and schools of fish
• Advantages
– Provides many eyes that can search for localized
food sources
– Confuses predators with sheer numbers
– Cooperation for hunting more effectively
FIGURE 26-13a Population distributions
(a) Clumped: a gathering of caterpillars.
Spatial Distributions
• Uniform distribution – constant distance
maintained between individuals; common
among territorial animals defending scarce
resources or defending breeding territories
• Examples: iguanas, shorebirds, tawny owls
• Advantage: a uniform distribution helps ensure
adequate resources for each individual
FIGURE 26-13b Population distributions
(b) Uniform: creosote bushes in the desert.
Spacial Distributions
• Random distribution – rare, exhibited by
individuals that do not form social groups;
occurs when resources are not scarce
enough to require territorial spacing
• Examples: Trees and other plants in rain
forests
FIGURE 26-13c
Population
distributions
(c) Random: trees
and other plants in
a rain forest.
Survivorship in Populations
• Survivorship describes the pattern of
survival in a population
• Life tables track groups of organisms
born at the same time throughout their life
span, recording how many continue to
survive in each succeeding year
Survivorship in Populations
• A survivorship curve for a population
can be produced by graphing life table
survivorship data
– Y-axis: the log of the number of individuals
surviving to a particular age
– X-axis: age
Survivorship in Populations
• Three types of survivorship curves can be
distinguished
– Late loss
– Constant loss
– Early loss
FIGURE 26-14b Life tables and survivorship curves
(b) Three types of survivorship curve are shown. Because the life
spans differ, the percentages of survivors (rather than ages) are
used.
Survivorship in Populations
• "Late loss" curves: seen in many animals
with few offspring that receive substantial
parental care; are convex in shape, with
low mortality until individuals reach old
age
– Examples: humans and many large
mammals
Survivorship in Populations
• "Constant loss" curves: an approximate
straight line, indicates an equal chance of
dying at any age
– Example: some bird species
Survivorship in Populations
• "Early loss" curves: high early mortality as
most offspring fail to become established;
are concave in shape
– Typical of most plants and many animals that
do not receive parental care
– Examples: most invertebrates and fish